Resistors play an important role in microelectronic circuits. A resistor is an electrical component designed to have an electrical resistance independent of the applied current or voltage. Two important issues in the fabrication of resistors in microelectronic processing are the accuracy of the individual resistor and the accuracy of the ratio of a pair of resistors.
Many microeletronic circuits, such as precision analog amplifiers, require the individual resistors to have a specific value to achieve the desired circuit performance level. Other circuits, such as differential amplifiers and analog-to-digital (ADC) or digital-to-analog (DAC) converter applications, require the accurate matching of two resistors, but do not require specific values for the resistors. “Matching resistors” means that the resistors have the same resistance value but may not be of any specific value due to manufacturing variations.
Because process variations affect matched pairs of resistors equally, high accuracy in matching resistors is easier to obtain in microelectronic fabrication processes than in setting individual resistor with a certain resistance value. But for some circuits, this degree of precision is still not adequate. For example, a resistance variation of merely 0.025 percent may compromise the linearity and accuracy of a 12-bit analog-to-digital converter. Thus maintaining the resistor-matching characteristics required for accurate performance is of paramount important in certain precision analog circuits.
Resistor trimming is the most common method used to adjust or match the resistors in these circuits. The term “resistor trimming” means the fine adjustment of the resistance of a resistor in a circuit, either to reach a particular resistance or to match a reference resistor. The resistors could be trimmed by various mechanical, electrical, or chemical methods. For example, a resistor can be trimmed by laser shaping, fuse blowing, or by changing the conductivity of the resistor by activation of impurity doping.
In these prior art resistor trimming methods, the trimming action can be extremely difficult to reverse. The adjustment of the resistance in these methods mostly can only occur in one direction. For example, by laser trimming, the resistance can only increase due to the shaping or the shaving of the resistance material. By activation of impurity doping such as metal migration, the resistance can only decrease due to the migration of the impurities into the resistance material.
Over the years, new resistor and conductor materials have been developed. Specifically, materials used in resistive memory cells have programmable resistance and exhibit reversible resistance change based on external influences. For example, materials having electric pulse-induced-resistive-change (EPIR) effect used in multi-bit non-volatile memory devices can exhibit reversible and repeatable resistance changes under the influence of electrical pulses. The EPIR effect can be found in thin film colossal magnetoresistive (CMR) materials such as Pr0.7Ca0.3MnO3 (PCMO). For detail of the EPIR effect, see S. Q. Liu, N. J. Wu, A. Ignatiev, “A new concept for non-volatile memory: the Electric-Pulse Induced Resistive change effect in colossal magnetoresistive thin film”, Proceedings of Non-Volatile Memory Technology Symposium 2001, San Diego, Calif., November 2001, p. 18–24. Other examples of a semiconductor resistive memory device are the metal-amorphous silicon-metal (MSM) electrical memory switch disclosed in U.S. Pat. No. 5,541,869 of Rose et al., and the electrical phase change materials such as TeGeSb, disclosed in U.S. Pat. No. 5,912,839 of Ovshinsky et al.
Thus it is advantageous to employ programmable resistance materials developed for resistive memory cells in resistor trimmer circuits for the advantage of reversibly trimmable resistance.